The instant invention relates to methods of producing biaxially-oriented polymer product and to the product produced thereby. More particularly, the instant invention is directed to methods of producing such product by continuous forging wherein polymer feedstock is advanced between a pair of opposed belts.
Biaxially-oriented polymers such as the polypropylene polymers sold under the trademark BEXOR have numerous advantages over unoriented polymers in that they have higher strength, improved stiffness, increased toughness and superior thermoformability. Biaxially-oriented polymers have been successfully produced using the techniques disclosed in U.S. Pat. No. 4,282,277, the disclosure of which is incorporated herein by reference, assigned to the assignee of the instant invention, wherein a tubular product is initially produced by hydrostatic extrusion using a mandrel in combination with a conical die. The tubular product is then cut, flattened and annealed utilizing expensive flattening and annealing equipment. While the mandrel-conical die approach results in excellent biaxial orientation of structures, it is necessary to expend considerable effort to remove residual curvature in the resulting sheet. Even when the curvature has been removed by reconfiguring the extruded tube into a flat sheet, residual and unbalanced stresses and strains in the original tube tend to subsequently re-assert themselves, resulting in difficulties when using the sheet.
In addition, the cost of the biaxially-oriented sheet produced in accordance with the teachings of U.S. Pat. No. 4,282,277 is relatively high since only a single billet may be extruded at a time and intermittent production results in high yield losses. In addition, it has proved expensive and difficult to keep the mandrel properly aligned within the die and produce uniform sheet during the extrusion process.
In order to avoid the expense of first forming a tube and subsequently slitting and flattening the tube to form a biaxially-oriented sheet, the inventors of the instant invention explored extruding polymers while in the solid state utilizing dies with protrusions therein as is set forth in U.S. patent application Ser. No. 806,994, filed Dec. 9, 1985, which application is the parent of the instant application. Utilizing protrusions within flat dies overcomes the non-uniformity which ordinarily occurs in flat dies due to friction between the workpiece and the die. While this approach results directly in a product which does not have to be slit and flattened before producing sheet, the process is an intermittent one wherein the sheets are produced one at a time as opposed to a continuous process. Moreover, utilization of a stationary die requires a very large machine, which can withstand required extrusion pressures on the order of 10,000 psi (70.3 kg/cm.sup.2) or more. Such a machine requires a considerable initial capital outlay.
While biaxially-oriented sheet can be produced by forging and cross-rolling, each of these processes have the drawback of being batch processes as opposed to continuous processes and have serious size and economic limitations. Consequently, it is difficult to produce elongated sheets of material. In addition, with a purely forged sheet, the sheet must be produced from a circular blank in order to have uniform biaxial orientation. The blank must then be trimmed which is both an additional step and a waste of material. While sheets produced by cross-rolling have an acceptable rectangular shape, they exhibit wavy surfaces resulting from non-uniform elastic springback of sheet emerging from the roll nip and consequently are not necessarily suitable for subsequent shaping and forming. Also, due to the short deformation time in cross rolling, the elastic springback results in a reduction of properties such as stiffness.
Recognizing the advantages of utilizing a continuous process for producing biaxially-oriented sheet, U.S. patent application Ser. No. 806,994 proposes an arrangement wherein a pair of converging belts, supported by rollers, continuously squeeze feedstock while biaxially orienting the feedstock to create biaxially-oriented sheet. However, support rollers which impart a convex surface to the belt are needed to compensate for the surface friction effect on the belt and the workpiece and produce a sheet uniformly biaxially oriented across the width.
Pursuing the opposed-belt concept, the inventors of the instant invention investigated utilization of a twin belt annealing press of the type generally utilized to manufacture items such as particle board, wherein wood particles or fibers are pressed together in the presence of a bonding agent to create sheets of material. This type of process is practiced at lower pressures than those required to biaxially orient polymers and was proved by tests to be unsatisfactory. The inventors also explored twin-belt machines used for producing metal sheet or slab by continuous casting. However, continuous casting machines only require sufficient pressure to retain the melt being processed thereby. In these types of machines, molten metal is deposited between a pair of converging belts and is spread laterally and longitudinally between the belts in order to continuously produce a metallic sheet product. In view of these approaches, there appeared to be little promise in utilizing opposed belt approaches for biaxially orienting feedstock to produce biaxially-oriented polymer sheet.